The present invention concerns a method and a device for guiding an aircraft during a low level flight.
A low level (or low altitude) flight phase, known as an LLF (Low Level Flight) phase, usually enables an aircraft to fly at low level, notably to follow the overflown terrain as closely as possible, in particular to avoid being picked out, at the same time as eliminating all risk of collision with a portion of the terrain. Such an LLF phase is generally situated at a predetermined terrain height, for example 500 feet (approximately 150 meters).
In particular, for a military aircraft, notably a military transport aircraft, one of the objectives of a low level flight is to benefit from the masking effect of the terrain to provide protection against threats in hostile geographical areas. To this end, in particular to carry out operations when flying on instruments under IMC (Instrument Meteorological Conditions), a three-dimensional reference trajectory is calculated in the usual way (taking into account the overflown terrain), and the aircraft is guided along this reference trajectory (either automatically by means of an automatic pilot system or manually by following information shown by a flight director).
The terrain used to calculate the reference trajectory is the terrain situated in a corridor, referred to hereinafter as the safety corridor, around the horizontal portion of the reference trajectory.
The width of the safety corridor is usually predefined to ensure the safety of the flight in IMC type flying, considering all flight conditions that could divert the aircraft from its reference trajectory.
Deviations with respect to the reference trajectory may be caused by faults in systems of the aircraft. The safety corridor is generally symmetrical on either side of the horizontal trajectory, deviations to the right or to the left of the trajectory being equally probable. The width of the safety corridor is the result of summing various error values that correspond to the performance of the various systems that contribute to the calculation of and to flying the trajectory. These are different causes liable to generate lateral deviations relative to the reference trajectory. There may be cited by way of example an error value relating to guidance, which his determined by measuring the maximum deviation considering all flight conditions and the faults affecting a guidance function, and an error value relating to the determination of the position of the aircraft, which is used to implement the guidance function. The guidance function (implemented via an automatic pilot system or a flight director) in fact relies on very precise and very reliable information as to the position of the aircraft.
If a fault is detected in one of the systems necessary for correct execution of an LLF operation, the crew is alerted and must trigger an escape maneuver, because continuing to fly under these degraded conditions is no longer safe. This escape maneuver comprises causing the aircraft to climb to a safety altitude at which there is no longer any risk of the aircraft colliding with the overflown terrain. During the escape maneuver, the aircraft usually continues to be guided laterally along the lateral (or horizontal) portion of the reference trajectory and a maximum rate of climb is applied to it. A portion of the safety corridor also serves to cover lateral deviations of the aircraft with respect to the reference trajectory during an escape maneuver (following a system fault, for example).
In particular, the escape maneuver may be initiated following a fault affecting one of the position sensors that enable the position of the aircraft to be generated. The minimum number of position sensors installed onboard the aircraft depends on the target safety level. To guide an aircraft in complete safety along a trajectory situated in a corridor, under IMC type flight conditions, at least two independent position sensors are necessary. Because in the aforementioned current procedure the aircraft must also be guided along the horizontal portion of the trajectory during the escape maneuver two position sensors are also necessary during this maneuver. Consequently, to allow LLF operations in IMC type flight, at least three independent position sensors must be provided (because two sensors are necessary for guidance during the escape maneuver and this maneuver may be triggered following a fault affecting one of these sensors).